Days 9 to 12 of gestation in the pig are marked by a pronounced asynchrony among littermate embryos. Previously published studies have suggested that the most advanced embryos within a litter by d 12 synthesize greater amounts of estrogen. Embryonic estrogen secretion has been shown to advance endometrial secretions, which may adversely affect less-developed littermates. To date, however, no comprehensive study of the developmental pattern and synthetic activities of individual littermate embryos during this period has been conducted. Litters were collected from Yorkshire gilts on d 9 (n = 11), 11 (n = 10), 12 (n = 5) and 13 (n = 8). Size (greatest diameter), DNA content (cell number), protein:DNA ratio and estrone (E1) and estradiol-17 beta (E2) content were determined for each embryo. Embryo sizes (mm greatest diameter) were (x +/- SEM) 1 +/- .1, 5.6 +/- .7, 41.2 +/- 11.7 and 405.7 +/- 16.7 on d 9, 11, 12 and 13, respectively. The daily variation in embryo size, expressed as CV was 82% on d 9, 145% on d 11, 206% on d 12 and 46% on d 13. DNA per embryo increased progressively from d 9 to 13, whereas the protein:DNA ratio declined. Content of E1 and E2 per embryonic cell was greatest on d 11 and d 12 before declining markedly on d 13. Cell number and embryo size were correlated positively in embryos 1 to 7 mm (P less than .01) and embryos greater than 100 mm (P less than .01) but not in embryos 8 to 100 mm.